1. Field of the Invention
The present invention generally relates to an inking device used for printing machines. The invention is in particular related to a so-called keyless inking device for which the regulation of amount of ink supplied to the surface of a printing plate in the direction of surface width is not required, for instance, an inking device which employs so-called short ink with high viscosity such as that used for offset printing.
2. Description of the Prior Art
With conventional keyless inking devices, the ink used for them is low in viscosity, and, as a device which can handle such ink easily, a mechanism is provided by which an ink fountain roller rotates with part of its circumferential surface immersed in ink stored in an ink pan to lead out the ink from the ink pan such devices are described in Japanese Patent Laid-Open Publications: No. 55-93456, `Ink Supplying Device for Rotary Printing Machine` (Prior Art Example 1). Further, a mechanism is provided in which ink is supplied directly onto the surface of an ink fountain roller from an ink nozzle, such as Japanese Utility Model Laid-Open Publications: No. 60-88741, `Ink Supplying Device` (Prior Art Example 2).
In the case in which so-called short ink with high viscosity is used as for offset printing, the fluidity of ink is very low so that the adhesion of ink to the circumferential surface of the ink fountain roller is not very good, and the movement of the ink to and through an ink nozzle is difficult. Conventional inking devices are, therefore, incapable of working with this type of ink. Under the circumstances, a new keyless inking device such as described in Japanses Utility Model Laid-Open Publication: No. 63-18242, `Ink Supplying Device`, (Prior Art Example 3), which is a keyless inking device with an ink pan, has come to be used widely.
In prior art devices described above, an ink fountain roller which rotates with part of its circumferential surface in contact with the ink in the ink pan is provided. For this ink fountain roller, the blade which forms the bottom plate of the ink pan is provided with its extremity positioned against the circumferential surface of the roller to form a gap from the circumferential surface of the roller along its width, and the blade is rotatable round its supporting point so as to be completely away from the roller. Further, a stirrer is provided to stir the ink in the ink pan. The form of this inking device is a modification of a conventional inking device to be used for printing machines which employ hard ink. It has a gap which is uniform to the direction of the roller width and which is provided between the extremity of a blade of the ink pan and the roller. It is also used as a keyless inking device. The thickness of the ink film in the direction of roller width is regulated uniformly by the gap between the extremity of the blade and the roller. In addition, the ink in the ink pan is stirred to assist in stirring the ink and in improving the adhesion of ink to the surface of the roller. When it is required to clean the inside of the ink pan for replacement of the ink such as in changing ink color, it is made simple and easy to clean the ink pan in a short time by separating the ink pan from the ink fountain roller.
The keyless inking device as described in Prior Art Example 3, which has an ink pan, is useful for using short ink. In keyless inking devices, however, an ink fountain roller and an ink transfer roller (a roller to receive and give ink) are mutually in contact, and, while the ink transfer foller rotates with a circumferential speed that is substantially the same as that of the plate cylinder, the ink fountain roller rotates with a circumferential speed which is smaller than the circumferential speed of the plate cylinder divided by a number 6 or 7. With this arrangement the state of contact in rotation of the ink fountain roller and the ink transfer roller is either direct or with an ink film in between, according to the behavior of the ink that is between both rollers. With the state of contact being either direct or with an ink film in between, the forces acting on the circumferential surfaces of the ink fountain roller and ink transfer roller vary, creating problems such as the generation of vibration and/or torsinal vibration in both rollers.
As a result, in a keyless inking device described in Prior Art Example 3, uneveness develops in the ink transfers from the ink fountain roller to the ink transfer roller and from the ink transfer roller to a roller (doctor roller) downstream of the ink transfer roller, and the thickness of the ink film cannot be kept constant, with the result of an excess supply of ink to the surface of the printing plate surface or a short supply of ink to the surface of the printing plate surface so that it is difficult to maintain a good quality of printing. The vibrations developed in the rollers often become very noisy, often making the work environment worse, and, furthermore, there are possibilities that the vibration developed in the roller can be transmitted to the whole of the printing machine and, extended use, can develop unexpected inconveniences.